This is the first part in a larger modularization of the data in `WP_Debug_Data`.
Previously this was a single massive method drawing in debug data from various
groups of related data, where the groups were independent from each other.
This patch separates the first of twelve groups, the `wp-filesystem` info,
into a separate method focused on that data.
This work precedes changes to make the `WP_Debug_Data` class more extensible
for better use by plugin and theme code.
Developed in https://github.com/wordpress/wordpress-develop/pull/7065
Discussed in https://core.trac.wordpress.org/ticket/61648
Props: afragen, apermo, costdev, dmsnell.
See #61648.
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